they already explained why.

there's a tick for 'include in progress' in the Duty Finder. that's what these people are hoping for.

you get a different message if a party is newly formed than when you enter a party in progress. if it's in progress you will get a message with the number of bosses already killed. for example 1/3 means the group in progress is about to do the 2nd boss.

what the people withdrawing are hoping for is to see 2/3
if they don't get that message, they withdraw.